Amoebae's most recognizable features include one or more nuclei and a simple contractile vacuole to maintain osmotic equilibrium. Food enveloped by the amoeba is stored and digested in vacuoles. Amoebae, like other single-celled eukaryotic organisms, reproduce asexually via mitosis and cytokinesis, not to be confused with binary fission, which is how prokaryotes (bacteria) reproduce. In cases where the amoeba are forcibly divided, the portion that retains the nucleus will survive and form a new cell and cytoplasm, while the other portion dies. Amoebae also have no definite shape.[7]

Genome

The amoeba is remarkable for its very large genome. The species Amoeba protea has 290 billion base pairs in its genome, while the related Polychaos dubium (formerly known as Amoeba dubia) has 670 billion base pairs. The human genome is small by contrast, with its count of 2.9 billion base pairs.[8]

Reaction to stimuli

Hypertonic and hypotonic solutions

Like most cells, amoebae are adversely affected by excessive osmotic pressure caused by extremely saline or dilute water. Amoebae will prevent the influx of salt in saline water, resulting in a net loss of water as the cell becomes isotonic with the environment, causing the cell to shrink. Placed into fresh water, amoebae will also attempt to match the concentration of the surrounding water, causing the cell to swell and sometimes burst.[9]

Amoebic cysts

In environments which are potentially lethal to the cell, an amoeba may become dormant by forming itself into a ball and secreting a protective membrane to become a microbial cyst. The cell remains in this state until it encounters more favourable conditions.[7] While in cyst form the amoeba will not replicate and may die if unable to emerge for a lengthy period of time.

Marine amoeba

Marine amoeba lack contractile vacuoles and their enzymes and organelles are not damaged by the salt water found in seas, oceans, salt swamps, salty rivers and ponds. Most are microscopic, but some can grow as large as grapes.[10]

From LoveToKnow 1911

AMOEBA, the Greek equivalent of the name
"Amibe" given by Bery St
Vincent to the Proteus
animalcule of earlier naturalists, used as a quasi-popular term for
any simple naked protist the sole external organs of which are
pseudopodia, i.e. temporary outgrowths of the clearer
outer layer of the soft protoplasmic body. It is also used as a
generic name, and in its present limitations by E. Penard includes
only those the pseudopodia of which are constantly changing, blunt
outgrowths. In the former wider sense, amoebae are found in
sluggish waters, fresh and salt,
all over the world; they readily make their appearance in infusions
putrefying after infection from aerially carried germs, and the
leucocytes or colourless blood corpuscles of Metazoa are
essentially amoebae in their structure and behaviour. The protoplasm of the
individual is divided into a centrally placed body, the nucleus, of relatively stable shape, and the cytoplasm,
itself divided into an outer, clearer ectoplasm ("ectosarc") and an
inner, more granular endoplasm ("endosarc"), passing into one
another. The movements of amoebae are of several kinds. (1) The
amoeba may grow out irregularly into blunt lobes, the pseudopodia,
some being emitted while others are retracted, and so may advance
in any direction by the emission of pseudopodia thitherward, and
the enlargement of these by the passage of the organism into them.
(2) Again, it may advance by a sort of rolling: the lower surface,
or that in contact with the substratum over or under which it is
passing, is viscid and adheres to the substratum, the superficial
dorsal layer passing forward and bending over to the ventral side;
whilst the converse action takes place at the hinder end; (3) or
again, the pseudopodia, when long, well marked and relatively
permanent, may serve as actual limbs on which the body is supported
and on which it moves. In the outgrowth of a pseudopod the process may take place
gradually, the ectoplasm growing as it stretches, or it may take
place by the limiting layer of the ectosarc bursting, as it were,
and a rounded prominence of the endosarc protruding and at once
forming a new "skin" or pellicle. This last mode, termed
"eruptive," is common in the case of the enormous, multinucleate
amoeba termed Pelomyxa palustris, which attains a
diameter when contracted and spherical of as much as a line (over 2
mm.). From the ease with which amoebae are obtained and kept alive
under the microscope,
as well as from their identity in structure with the primitive
elements of Metazoa, they have always been favourite objects of
study for protoplasmic physiology under its simplest conditions.
Among the investigators of protoplasmic movements we may cite F.
Dujardin, O. Biitschli, L. Rhumbler and H. S. Jennings. The opening
to the exterior of the contractile vesicle has been found here.
Pelomyxa has yielded to A. E. Dixon and M. Hartog a peptic ferment, such as has
been extracted by C. F. W. Krukenbergfromthe Myxomycete
Fuligo (Flowers of Tan), which is the largest known naked
mass of protoplasm without cellular differentiation.

Amoeba shows also the multiplication by fission, so
characteristic of the cell: for
the study of other modes of reproduction, spore formation and.
syngamic (or so-called fertilization) processes, fresh-water or
salt-water amoebae are ill suited, and up to this date we do not
know the life cycle of any
free-living naked amoeba, though that of some parasitic forms and
shell-bearers have been fully
made out. Some amoebae are certainly young states of Myxomycetes.
Encystment, the excretion of a membrane around the cell to tide over unfavourable circumstances,
has been noted in almost all species.

Amoeba coli and A. histolytica are parasites
in the gut of man, the former relatively harmless, the latter the
cause of severe dysentery and hepatic abscess, common in India.

H. S. Jennings has recently made a full study of the movements
of Amoeba, and of its general behaviour, and found therein
many indications that these are on the whole such as we should
expect of an organism working by "trial and error" rather than the
uniform modes of non-living beings. Thus the operations of an
amoeba ingesting a round, encysted Euglena are summed up
thus: "One seems to see that the amoeba is trying to obtain this
cyst for food, that it shows remarkable pertinacity in continuing
its attempts to put forth efforts to accomplish this in various
ways, and that it shows remarkable pertinacity in continuing its
attempts to ingest the food when it meets with difficulties. Indeed
the scene could be described in a much more vivid and interesting
way by the use of terms still more anthropomorphic in tendency."
(M. H A.)

Name

References

links

Vernacular
names

Amoeba (plural = amoebae) is well-known as a unicellular organism, a protist. The amoeba was first discovered by August von Rosenhof in 1757.[1] It is a genus of protozoa that moves with false feet, called pseudopodia.

Contents

Pseudopodia

These are unique extensions of the organism's membrane. They are used by the amoeba for phagocytosis (active food/nutrient intake) and motility (self-propelled movement).

Life

Amoebae are often found within freshwater, typically on vegetation in decay in still or slow moving water, or in the benthic zone of some lakes. However, they are common organisms of study because it is easy to keep them in a laboratory. They are used to study protozoa and to demonstrate cell structure and function.